1. Field of the Invention
The present invention relates to a method and an apparatus for producing tofu automatically.
2. Description of the Related Art
In Japan, tofu (soybean curd) has been manufactured from ancient times. In a traditional method of tofu production, soaked soybeans are first added with water and ground to provide a soybean slurry. The resulted soybean slurry is then heated and squeezed out from a cloth filter to provide hot soybean milk. Thereafter, the hot soybean milk is added with a coagulant and stirred to initiate the solidification of the milk. The good and bad of the solidification technique produce a large difference in quality of tofu. The solidification process is thus quite important for the production of tofu.
Recently, retort packed tofu, developed for mass production of tofu, has taken the place of the traditional tofu production method which requires an artisan""s skill. Retort packed tofu is of the type called Kinu-tofu, because it has a smooth taste like silk as represented by the Japanese term Kinu. For producing Kinu-tofu, cold soybean milk, mixed uniformly with a coagulant, is filled in a container and sealed. Thereafter, it is heated to a high temperature for solidification.
When coagulants such as a natural bittern (main components being magnesium chloride and calcium chloride) and quick-acting, formulated magnesium chloride and calcium chloride are used, it is difficult to restrain a solidifying reaction for a long time, even though the soybean milk is kept cold. It may often happen that the solidifying reaction progresses before the soybean milk is filled into a container or a shaping box, in which instance the end product tofu, though looks solidified nicely, becomes collapsible and presents a rough taste instead of a desired bouncy and smooth one.
In recent years, coagulant adding methods called in-line mixing methods have been proposed in which a coagulant solution is poured directly into a line and mixed with cold soybean milk flowing through the line. These methods are advantageous in that they significantly shorten the time from the addition of the coagulant solution to the filling of the soybean milk into a container. However, the amount of coagulant added during the shortened time is as little as 0.1 to 2.0 wt. % per 100 g of soybean milk. If the coagulant solution added to the soybean milk is not in a high concentration of 30% or more, protein in the milk begins solidifying, thereby causing the resultant tofu to have a rough taste. If the coagulant is added in excess, the resulting tofu will have astringency and/or bitterness which adversely affect a targeted flavor thereof. Thus, the amount of the added coagulant must be controlled precisely but this is difficult to achieve with the proposed in-line mixing methods.
In addition, unless the coagulant added to the soybean milk is uniformly dispersed in a short period of time, there arises variations in the concentration of the coagulant, which act as a bar to the manufacture of homogenized tofu products. The concept of continuously adding a coagulant into a line of tofu production and mixing them therein has not been so far applied to a line of mass production of tofu in an effective way. Failure to realize such an application is due to the unreliability of a system for adding a coagulant to soybean milk and stirring them.
In a continued research to overcome the foregoing problems, the present inventors have conceived a new method of automated production of tofu, in which after continuously added with a coagulant in a line of tofu productions, soybean milk is stirred by an in-line mixer, and then the concentration of the added coagulant in the soybean milk is detected for controlling a coagulant supply apparatus based on the detected coagulant concentration. The method further continues with controlling the coagulant supply apparatus on the basis of the rate of flow of the soybean, followed by stirring the soybean milk by the in-line mixer. The inventors have perceived that the conceived method can achieve the precision addition and mixing of a coagulant to and with the soybean milk, and this had lead to the present invention.
According to a first aspect of the present invention, there is provided a method for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which method comprises the steps of: stirring said soybean milk, after added with the coagulant, in the soybean milk transfer circuit; detecting the concentration of the coagulant in the soybean milk; and controlling the amount of addition of the coagulant on the basis of the detected concentration.
The step of detecting the coagulant concentration may be performed by using one of an electric conductivity measuring method and an ion measuring method. From the standpoint of precision and response capability, the electric conductivity measuring method is preferred.
Preferably, the method further comprises the steps of: measuring the electric conductivity of the soybean milk before and after added with the coagulant; and controlling the amount of addition of the coagulant on the basis of a difference between the detected conductivity values. This enables the desired control with increased precision. The difference in conductivity may be calculated by a calculator.
According to a second aspect of the present invention, there is provided a method for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which method comprises the steps of: measuring the flow rate of the soybean milk; controlling the amount of addition of the coagulant in accordance with the detected flow rate value; and stirring the soybean milk added with the coagulant.
A soybean milk flow rate detector may be employed for measuring the flow rate of the soybean milk. The soybean milk flow rate detector is desirably disposed upstream of a coagulant adding section so that it can conveniently detect only the flow rate of the soybean milk and adjust a time lag in adding the coagulant.
According to a third embodiment of the present invention, there is provided a method for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which method comprises the steps of: measuring the flow rate of the soybean milk; controlling the flow rate of the soybean milk in accordance with the detected flow rate value; controlling the amount of addition of the coagulant in accordance with the detected flow rate value; and stirring the soybean milk added with the coagulant.
With this arrangement, it becomes possible to control the flow rate of the soybean milk and the amount of addition of the coagulant at the same time.
According to a fourth aspect of the present invention, there is provided an apparatus for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which apparatus comprises: a stirring apparatus disposed in the soybean milk transfer circuit for stirring the soybean milk to uniformly disperse the coagulant added to the soybean milk; coagulant supply means placed in fluid communication with the stirring apparatus for adding the coagulant to the soybean milk being transferred through the circuit; a coagulant concentration detector disposed downstream of a coagulant adding section for detecting the concentration of the coagulant in the soybean milk; and a coagulant concentration adjuster for controlling, on the basis of a detected signal from the coagulant concentration detector, the amount of addition of the coagulant supplied from the coagulant supply means.
An additional coagulant concentration detector may be disposed upstream of the coagulant adding section for detecting the concentration of the coagulant in the soybean milk. The coagulant concentration detector may comprise an electric conductivity detector for measuring the electric conductivity of the soybean milk added with the coagulant.
According to a fifth aspect of the present invention, there is provided an apparatus for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which apparatus comprises: a stirring apparatus disposed in the soybean milk transfer circuit for stirring the soybean milk to uniformly disperse the coagulant added to the soybean milk; a soybean milk flow rate detector disposed at that part of the soybean milk transfer circuit positioned upstream of the stirring apparatus, for detecting the flow rate of the soybean milk; coagulant supply means placed in fluid communication with the stirring apparatus for adding the coagulant to the soybean milk being transferred through the circuit; and a coagulant flow rate adjuster for controlling, on the basis of a detected signal from the soybean milk flow rate detector, the coagulant supplied from the coagulant supply means.
According to a sixth aspect of the present invention, there is provided an apparatus for producing tofu by continuously adding a coagulant to soybean milk in a soybean milk transfer circuit, which apparatus comprises: a stirring apparatus disposed in the transfer circuit for stirring the soybean milk to uniformly disperse the coagulant added to the soybean milk; a soybean milk flow rate detector disposed at that part of the transfer circuit upstream of the stirring apparatus for detecting the flow rate of the soybean milk; a soybean milk flow rate adjuster for controlling, on the basis of a detected signal from the soybean milk flow rate detector, the flow rate of the soybean milk to remain constant; coagulant supply means placed in fluid communication with the stirring apparatus for adding the coagulant to the soybean milk being transferred through the circuit; a coagulant flow rate detector disposed on the coagulant supply means for detecting the amount of supply of the coagulant; and a coagulant flow rate adjuster for controlling, on the basis of a detected signal from the coagulant flow rate detector, the amount of addition of the coagulant supplied from the coagulant supply means.